We actually had a customer call in with that exact problem today... but I am guessing it may have been you
At 5 volts, 20 amps is actually pretty common, especially if you are running Putnam brushes or the Black Magic brushes, both draw a lot of amps. There is no reason that should blow up the GFX though... the GFX should error out saying "motor amps too high." I would send it in asap and they will get you fixed up quickly.
For future reference, I would recommend not running the motor at more than 3 volts
If a motor draws 20 amps at 3 volts, then you have some serious HP, or something is binding up.